1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly to a data transmission method using packet aggregation in order to improve resource efficiency in a multi-carrier based wireless communication system.
2. Description of the Related Art
Generally, a communication system uses a hierarchical frame structure in order to efficiently transmit data of an upper layer. Specifically, a wireless communication standard such as an Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard and an IEEE 802.16 standard has conducted research and development for frame aggregation in a Media Access Control (MAC) layer and a physical (PHY) layer in order to maximize the efficiency of radio resources.
FIG. 1 illustrates a general framing process in a wireless communication system. As illustrated in FIG. 1, a MAC layer receives a MAC Service Data Unit (MSDU) 110 and attaches a MAC header 111 to the MSDU 110, thereby constructing a MAC Protocol Data Unit (MPDU) 11. The MAC header 111 includes various information such as Source Addresses (SAs) and Destination Addresses (DAs). The MSDU 110 may be classified according to traffic characteristics and identified by Traffic Identifiers (TIDs). The MPDU 11 is a PHY Service Data Unit (PSDU) 120 and is transferred to a PHY layer. Then, a PHY header 121 is attached to the PSDU 120, so that a PHY Protocol Data Unit (PPDU) 12 is constructed.
The PHY header 121 includes parameters for determining a transmission scheme including a modulation and coding scheme, etc. Before the PPDU 130 is transmitted wirelessly, a preamble 131 including channel estimation and synchronization information is attached to the PPDU 130.
In the framing process as described above, a data aggregation method may include an MSDU aggregation method and an MPDU aggregation method.
FIG. 2 illustrates the conventional MSDU aggregation method, in which MSDUs 210, 220 and 230 having the same DA 1 and TID1 construct an MPDU 21, an MPDU 22 and an MPDU 23, respectively, together with each MAC header 215. In this process, two of the three MAC headers 215 are removed, so that resource efficiency can be improved. This scheme has a greater effect when traffic of a specific connection session is very bursty. However, for aggregation of the MSDUs 210, 220 and 230, buffering is required until a predetermined number of MSDUs are accumulated. Therefore, it is difficult to apply this scheme to real-time traffic because jitter and delay variation increase.
Further, this scheme can achieve a good effect when short packets are aggregated. However, because an MPDU generally has a maximum length within 2000 to 3000 bytes, this scheme has reduced performance when long packets are aggregated.
FIG. 3 illustrates the conventional MPDU aggregation method. In which, DAs 315, 325 and 335 and TIDs 310, 320 and 330 are different from each other, but MPDUs 31, 32 and 33 having various DAs can construct and transmit one PPDU 130. Accordingly, the MPDU aggregation method is mainly effective in an Access Point (AP). In the MPDU aggregation method, because transmittable MPDUs are aggregated regardless of DAs and TIDs the instant a transmission opportunity is obtained, it does not influence jitter and delay time.
However, because it is not possible to load information regarding a modulation scheme and a transmission scheme for all receive terminals into one PHY header, it is necessary to set the transmission rates of the receive terminals to be the same, particularly, the transmission rate of a terminal having the worst channel conditions from among the receive terminals, even though the channel conditions of the receive terminals are not identical. Therefore, the waste of resources may be great.
Further, because all terminals must receive not only their own MPDUs but also MPDUs for terminals having different destinations, resources may be wasted. In addition, because all terminals must refer to the headers of all MPDUs in order to identify MPDUs targeting themselves, a processing delay may increase.